Method for measuring plate thickness

ABSTRACT

A method for measuring an increase or decrease in the thickness of a transparent glass or plastic plate, by differentiating between the convergence and divergence of an interference fringe pattern caused by the increase or decrease in the thickness of the plate when it is moved passed a testing station. The interference fringe pattern is created by directing a nonparallel pencil or wedge of rays on the plate. The resulting two sets of rays being reflected by the front and rear surfaces of the plate are directed and focused to a light detector. By detecting the convergence or divergence of the fringe pattern and counting the fringe rings passing the detector, the change in thickness of the plate is measured.

States atent n 1 lKasahara et al.

[ METHOD FOR MEASURING PLATE OTHER PUBLICATIONS THICKNESS Wavelength orLength Measurement by Reversible [75] Inventors: Ichiro Kasahara,Mitaka-shi, Tokyo; Fringe Counting JOSA; Vol. 43, N0. 6, pg. 505t.

Hideki Yamaguchi, Toshima-ku, Tokyo, both of Japan PrimaryExaminer-Ronald L. Wibert Assistant Examiner-C0nrad Clark [73] Assignee.Nihon Denskl Kabushikl Kaisha, Atmmeywebb Burden Robinson & Webb Tokyo,Japan 22 Filed: Feb. 25, 1970 ABSTRACT [2]] A l. M 13,976 A method formeasuring an increase or decrease in the thickness of a transparentglass or plastic plate, by differentiating between the convergence anddiver- [52] US. Cl ..356/l08 genes of an interference fringe patterncaused by the [51] '9 "Golb 9/02 increase or decrease in the thicknessof the plate when [58] Field of Search ..250/219 TH; it is moved passeda testing Station The interference 356/1064 frin e attern is created bdirectin a nonaralle'l B P Y E P pencil or Wedge of rays on the plate.The resulting two [56l References C'ted sets of rays being reflected bythe front and rear sur- UNITED STATES PATENTS faces of the plate aredirected and focused to a light detector. By detecting the convergenceor divergence 3,319,515 5/1967 Flournoy .356/l08 of the fringe patternand counting the fringe rings 3,549,870 12/1970 Lay ..356/106 X passingthe detector, the change in thickness of the 2,848,921 8/1958KOLlllkOVllCh ..356/l06 plate is measured 3,409,375 11/1968 Hubbard..356/l06 3,354,311 11/1967 Vali et al......... ....356/106 6 Claims, 12Drawing Figures 2 4 WAVE-FORMING WAVE-FORMING J CIRCUIT CIRCUIT U U1:

, I f i 6 9 41 l l 13 l )4 6 t I AT" 7 t za DIFFERENTIATING PHASECIRCUIT INVERTER )7 j GATE c|Rcu|r- 5 1 GATE CIRCUIT REVERSIBLE comm rPATENTEDuAmma 3,720,471

SHEET 10F 5 WAVE-FORMING WAVE-FORMING cmcm r cmcun )2 44 ,0 f J mFFsaiur l xgmg 1 3% GATE cmcun /)7 \GATE CIRCUIT REVERSIBLE COUNTER-P 0PATENTEUMARI 31975 SHEET 5 [IF 5 METHOD FOR MEASURING PLATE THICKNESSThis invention relates to a method for measuring the thickness oftransparent; for example, glass or plastic plates. More particularly, itrelates to a method of optically measuring the thickness of the plates.

One manner of measuring plate thickness is to use a micrometer. Thismethod is unsatisfactory, because it is impossible to measure the platethickness continuously. Further, the measurement precision of amicrometer is limited to about 1 micron.

interferometers have been used to measure plate thickness with ameasurement precision in the order of one wavelength. Even so, parallellight rays have been irradiated on the plate resulting in a parallelinterference fringe pattern. This pattern, permits the relativevariation of the plate thickness to be measured by a light detector,such as a phototransistor, or counting the number of fringes movingacross the detector. This interferometer technique has a drawback:fringes move in one direction only, making it impossible to determinewhether the variation in plate thickness is a decrease or increase.

In present invention, either convergent or divergent rays are used forirradiating the plate. As a consequence, a ring, eliptical or parallelfringe pattern is produced by interference of the two rays reflected bythe front and rear'surfaces of the said plate. Since the fringes in thepattern have a divergent movement, i.e., an outward movement from cehterto periphery, when the thickness of the plate increases and a convergentmovement, i.e., an inward movement from periphery to center, when thethickness of the plate decreases, it is possible to determine increaseordecrease of the plate thickness.

Briefly according to this invention, a transparent plate is movedrelative to a testing station. A pencil or wedge of non-parallelcoherent rays are directed at the plate. The resulting two sets of raysare directed and focused at a light detector to create an interferencefringe pattern. As the thickness of the plate passing the test stationincreases or decreases, the interference fringe pattern diverges orconverges. The divergence or convergence of the fringe pattern ismeasured at the light detector and the fringe rings are counted movingacross the light detector.

According to a preferred method of this invention,

I aberration of the fringe pattern resulting, for example,

from a portion of the plate having non-parallel faces is corrected bymodulating the rays entering the light detector with a constantmodulating frequency and comparing the phase of the modulated output ofthe light detector with the modulating frequency to detect aberration ofthe fringe. ,The fringe pattern is then refocused if necessary on thelight detector.

It is an advantage of the present invention that not only an absolutechange in thickness of the plate is detected, but the direction of thechange; that is, increasing or decreasing thickness is determined. It isanother advantage of this invention that the fringe center aberrationmay be corrected providing more accurate readings.

Various other objects and advantages of this invention will becomeapparent from the following detailed description made with reference tothe accompanying drawings in which:

FIG. 1 shows one embodiment of the invention in which a convex lens isused for producing a'divergent pencil of rays.

FIG. 2 shows a ring fringe pattern produced by the embodiment shown inFIG. 1.

FIG. 3 shows the wave forms produced by the various electrical unitsconstituting the embodiment shown in FIG. 1.

FIG. 4 shows a second embodiment according to the present invention inwhich a convex lens is used for producing a convergent pencil of rays.

FIG. 5 shows a third embodiment of the invention in which a concave lensis used for producing a divergent pencil of rays. a

FIG. 6 shows a fourth embodiment of the invention in which a cylindricallens is used for producing a divergent wedge of rays.

FIG. 7 shows a parallel fringe pattern produced by the fourthembodiment.

FIG. 8 shows a fifth embodiment of the invention in which a means, inthe form of a vibrating half mirror, has been incorporated forcorrecting fringe center aberration.

FIG. 9 shows a three-dimensional view of a mirror box used in the fifthembodiment.

FIG. 10 shows wave forms produced by the various electrical unitsconstituting the embodiment shown in FIG. 8.

FIG. 1 I shows a sixth embodiment of the invention in which a means, inthe form of a vibrating parallel glass plate, has been incorporated forcorrecting fringe center aberration.

FIG. 12 shows a seventh embodiment of the invention in which thecorrecting means for fringe center aberration takes the form of anelectro-optical element.

Referring to FIG. 1, a glass plate 1 is moved unidirectionally onrollers 2 at a constant speed. Coherent rays generated by a laser 3,such as a He-Ne laser, are reflected by a reflector 4 so as to passthrough a convex lens 5 at the focal point of which the rays arediverged and irradiated on the glass plate 1.

A portion of the rays irradiating the plate are reflected by the platesfront surface. The remaining portion penetrate the front surface of theplate, a portion of which are reflected by the rear surface of theplate. As a result, an interference fringe pattern is produced by thetwo sets of rays. If a screen (not shown) is inserted in the light path6, a ring fringe pattern as shown in FIG. 2 will appear on the screen.

Now even if the vicinity of the center of the fringe is observed, thedistance between plate 1 and the screen and between the lens 5 and plate1 is immaterial as far as the fringe pattern is concerned, 'but theplate thickness is quite pertinent. The fringe intensity is maximum whenthe following equation is satisfied:

where n is the refractive index of the plate, d is the plate thickness,A is a wave length of the rays irradiating the plate and N is aninteger. Therefore, the variation in plate thickness is known by thevariation of N. Further, the increase or decrease of plate thickness isascertained according to fringe divergence or convergence respectively.

The two sets of rays reflected by the front and rear surfaces arereflected by a half mirror 7, positioned in the light path 6, and passthrough a convex lens 8. The center portion of the rays passing throughthe convex lens 8 is reflected by a reflector 9 so as to enter aphototransistor 10. On the other hand, the outer, peripheral portion ofthe said rays enters a phototransistor 11. The intensity of the raysdetected by either of the phototransistors is increased or decreasedaccording to the phase difference of the light waves of the two sets ofrays reflected by the front and rear surfaces of the plate.

Now, in the event that the ring fringe pattern diverges due to anincrease in the thickness of the portion of the plate being irradiatedby the divergent pencil of rays, a fringe signal as shown in FIG. 3A isdetected by the phototransistor 10 and a fringe signal as shown in FIG.3B detected by the phototransistor 11. The respective phases of the twofringe signals are different, the difference being made about 1'r/2. Thephase of the signal detected by the phototransistor 10 leads that of thesignal detected by the phototransistor 11.

The signal detected by the phototransistor 10 is used as a gate signaland is converted into a signal as shown in FIG. 3C by a wave-formingcircuit 12. The output of the wave-forming circuit 12 is converted intopulses as shown in FIG. 3D by a differentiation circuit 13. The signaldetected by the phototransistor 11, on the other hand, is converted intoa signal as shown in FIG. 3E by a second wave-forming circuit 14. Theoutputs of the differentiation circuit 13 and the wave-forming circuit14 are fed into a gate circuit 15. When the two signals aresimultaneously positive, a series of pulses as shown in FIG. 30 areproduced by the gate circuit 15.

The output of the wave-forming circuit 14 is also fed into a phaseinverter 16 where it is inverted to produce a signal as shown in FIG.3F. The outputs of the differentiation circuit 13 and the phase inverter16 are fed into a gate circuit 17. Since, in this case, the two signalsare at no time simultaneously positive, no signal is produced by thegate circuit 17 (FIG. 3H).

In the case that the fringe converges with the movement of the glassplate 1, a fringe signal as shown in FIG. 3A is detected by thephototransistor 10, the phase of the said signal lagging behind that ofthe signal as shown in FIG. 3A. However, the phase of the signaldetected by the phototransistor 11 (FIG. 3B) neither leads nor lags thatof the signal shown in FIG. 3B. As a result, a series of pulses isproduced by the gate circuit 17 (FIG. 3H and no signal is produced bythe gate circuit (FIG. 30).

The outputs of the two gate circuits are fed into a reversible counter18, where the output pulses are counted. The output pulses produced bythe gate circuit 15 are plus-counted and the output pulses produced bygate circuit 17 are minus-counted. A recorder (not shown) connected tothe reversible counter continuously records these plus and minus pulsesaccording to the increase and decrease in plate thickness respectively.

In the embodiment just described, the pencil of rays produced by theconvex lens irradiates the surface of the plate divergently. In thesecond embodiment shown in FIG. 4, however, the pencil of rays producedby the convex lens 5 irradiates the surface of the plate 1 convergently.In this case, the ring fringe pattern is produced by the two sets ofrays reflected by the front and rear surfaces of the glass plate. Themovement of the glass plate causes each fringe of the said fringepattern to move convergently or divergently.

In the third embodiment shown in FIG. 5, the pencil of rays is producedby a concave lens 20, the rays being irradiated on the plate surfacedivergently. In this case also, the ring fringe pattern is produced bythe two sets of rays reflected by the front and rear surfaces of theglass plate 1. Again the movement of the glass plate causes each fringeof the said fringe pattern to move convergently or divergently.

In the fourth embodiment shown in FIG. 6, a wedge of rays is produced bya cylindrical lens 21, the rays being unidirectionally irradiated on theplate surface either divergently or convergently. In this case, aparallel fringe pattern as shown in FIG. 7 is produced by the two setsof rays reflected by the front and rear surfaces of the glass plate. Themovement of the glass plate causes each fringe of the said fringepattern to symetrically move convergently or divergently with respect toline 0-0 as the center. Fringe convergence and divergence isdifferentiated by the same arrangement of electrical units as shown inFIG. 1.

It is possible to use other types of lenses in order to produce adivergent or convergent pencil of rays. For example, a cylindrical lenswith one side concave can be used for the divergent rays. Again, the useof an eliptical lens is feasible. In this case, the two sets of raysreflected on the front and rear surfaces of the plate would produce aneliptical fringe pattern. The movement of the glass plate causes eachfringe of the said fringe pattern to move convergently or divergently.

In the above described embodiments, the light intensities of the fringesare detected by the two phototransistors and the respective phases ofthe fringe signals detected by the said transistors are different.Further, the phase difference between the two signals must be keptconstant in order to precisely differentiate between the convergent anddivergent movements of the fringe. In the case when the front surface ofthe glass plate is not parallel to the rear surface, however, thecentral point or the center line of the fringe pattern is moved. As aresult, the phase difference between the two signals cannot be keptconstant and it is, therefore, impossible to differentiate between theconvergent and divergent movements of the fringe pattern precisely.

In the fifth embodiment, the light signals which enter thephototransistors are modulated by a vibrating half mirror 30. Referringto FIGS. 8 and 9, the half mirror 30 is arranged in a mirror box 31 andsupported by a mirror supporter 32. The said mirror supporter is, inturn, supported by two rods 33, 34 which are fixed to the box 31 so thatthe half mirror 30 turns on the rods freely. The mirror supporter isarranged between two magnets 35 and 36, having opposite polarities.Conductors 37 on which coils 38 are wound are fitted to two oppositeends of the mirror supporter, the said coils being contiguous to themagnets 35 and 36. The coils 38 are connected to an oscillator 39 whichsupplies an alternating current. As a result, the half mirror 30 is madeto vibrate in accordance with the fluctuations of the said current.

In this case, the phototransistor is connected to the wave-formingcircuit 12 via a low frequency band pass filter 40 and thephototransistor 1 1 is connected to the wave-forming circuit 14 via alow frequency band pass filter 41 in order to differentiate between theconvergent and divergent movements of the fringe pattern without beingaffected by the aforementioned modulation. Generally, the frequency ofthe fringe signal is less than lOHz since the plate thickness does notvary abruptly and moreover, since the frequency of the alternatingcurrent supplied by the oscillator 39 is extremely high compared withthe frequency of the fringe signal, the outputs of the two low-passfilters 40, 41 consist (as shown in FIG. 10A) of the fringe signal only.

The modulated fringe signal detected by the phototransistor 10 is fedinto a gate circuit 42 and passes through the said gate circuit duringthe time that the pulses, as shown in FIG. 10B, are applied, said pulseshaving a limited time, being applied to the gate circuit 42 from thewave-forming circuit 12. FIGS. 10C, D and E show the signals passedthrough the gate circuit 42. In this embodiment, therefore, fringecenter aberration is corrected only when the fringe center is bright.

When the fringe center is bright and the phototransistor l0 detects theleft side slope of the waveform, the fringe signal is modulated by theAC frequency f provided by the oscillator 39 (FIG. 10C). In this case,the phase of the modulated signal coincides with the phase of said ACfrequency f, and as a result, a plus signal is produced by the phasedetector 43. The output of the phase detector 43 is in turn fed into anamplifier 44, the amplified output of which is amplified to a motor 45connected to a reduction gear 46 by a shaft 47. The reduction gear 46 islinked to the mirror box 31 by a shaft 48. By means of this arrangement,the mirror box 31 is rotated either clockwise or anticlockwise until thephototransistor l0 detects the maximum intensity of the fringe signal.

When the phototransistor 10 detects the top of the waveform (see FIG.10D), however, the fringe signal is modulated by frequency 2f, and theoutput of the phase detector is zero, In this case, the mirror box 31does not rotate.

When the phototransistor 10 detects the right side slope of the waveform, the fringe signal is modulated by frequency f (see FIG. 10E) whosephase lags behind the phase of the alternating current provided by theoscillator 39 by 1r. As a result, a minus signal is produced by thephase detector 43, the said signal being amplified by amplifier'44 andapplied to the motor 45. In this case, therefore, the mirror box 31 isrotated but in the opposite direction to that when the phototransistor10 detects the left side slope ofthe waveform.

6 periphery of the center. As a consequence of the above, it isdesirable to narrow the width of the pulse supplied by the wave-formingcircuit 12.

FIG. 11 shows the sixth embodiment which is a modification of the fifthembodiment. In this embodiment, a parallel glass plate 50 is arranged inthe optical path between a stationary half mirror 51 and light detectors(not shown). With this arrangement, it is possible to rotate the saidplate in the same manner as the half mirror shown in FIG. 9. The opticalpath passing through the parallel glass plate 50 moves parallel inaccordance with the inclination on the said plate. As a result, thesignals detected by the light detectors are modulated.

FIG. 12 shows the seventh embodiment which again is a modification ofthe fifth embodiment. In this embodiment, an electrooptical element 52,such as a KDP or ADP, is arranged in the optical path between astationary half mirror 53 and light detectors (not shown). The directionof the optical path passing through the electro-optical element 52changes in accordance with the voltage applied across the said element.An AC source 54 is used for modulating the fringe signal and a variableDC source 55 is used for correcting the aberration at the fringe center.

Having thus defined our invention in detail and with the particularityrequired by the patent laws, what is desired protected by LettersPatents is set forth in the following claims.

We claim:

1. A method for measuring the change in thickness of the transparentplate continuously moving relative to a light detector comprisingcentral and peripheral detectors for detecting the central and one otherfringe of an interferometer diffraction pattern produced by nonparallelcoherent rays reflected from the surfaces of the transparent platecomprising the steps for:

1. irradiating the said plate with non-parallel coherent rays;

2. directing and focusing the resultant two sets of rays which arereflected by the front and rear surfaces of the said plate to the saidlight detector, the said two sets of rays producing interference fringepattern at said detector which diverges or converges in accordance withthe increase or decrease of the plate thickness;

3. modulating the rays entering the said light detector central detectorby a slight back and forth deflection at a constant modulation frequencyaccording to a modulation signal;

4. detecting and comparing the modulated fringe signal with themodulation signal and producing a control signal indicative of the phaseshift which corresponds to the aberration at the center of the fringepattern from the central detector;

5. in response to said control signal mechanically driving means forfocusing and correcting the aberration at the center of the fringepattern;

6. differentiating between the divergent and convergent movement of thefringe pattern by comparing the fringe signals detected by the centraland peripheral light detectors; and,

7. reversibly counting the number of fringes moving across the saidlight detector.

2. A method for measuring the thickness of the trans parent plateaccording to claim 1 wherein the rays entering the light detector aremodulated by vibrating a half mirror arranged in the path of the saidrays.

3. A method for measuring the thickness of the transparent plateaccording to claim 1 wherein the rays entering the light detector aremodulated by a vibrating parallel glass plate arranged in the path ofsaid rays.

4. A method for measuring the thickness of the transparent plateaccording to claim 1 wherein the rays entering the said light detectorare modulated by an electro-optical element arranged in the path of saidrays.

5. The method according to claim 1 wherein the differentiation betweenthe divergence and convergence movement of the fringe pattern is made bydetecting with a first phototransistor (10) the intensity of the rays atthe center of the fringe pattern and with the second phototransistor(11) the intensity of a peripheral portion of the fringe pattern, thephototransistors being arranged to produce signals of the same frequencyspace by about 1-r/2, feeding the signal created by the firstphototransistor (10) to a first wave-forming circuit (12) for creating apositive pulse output during a portion of the positive part of the inputsignal, feeding the output of the first wave-forming circuit (12) to adifferentiation circuit (13) for creating a short positive or negativesignal respectively at the start and finish of the pulse output of thefirst wave-forming circuit, simultaneously feeding the signal created atthe second phototransistor (11) to a second wave-forming circuit (14) toform a pulse output during a positive portion of part of the inputsignal such that said pulse overlaps the output pulse of the firstwave-forming circuit, feeding the output of the first differentiationcircuit (13) and the second wave-forming circuit (14) to a first gatecircuit (15) which produces a series of pulses when the inputs theretoare simultaneously positive, which occurs during divergence of theinterference fringe as the output signal of first phototransistor leadsthe output signal the second phototransistor, feeding the output of thesecond wave-forming circuit to a phase inverter (16), feeding the outputof the first differentiation circuit (13) and the phase inverter (16)simultaneously to a second gate circuit (17), which produces a series ofsignals when the inputs thereto are simultaneously positive, whichoccurs during convergence of the interference fringe pattern as theoutput signal of the first phototransistor drags behind the outputsignal of the second phototransistor and feeding the outputs of thefirst and second gate circuits (15, 17) to a responsive counter (18)wherein the output pulses of the first gate circuit are counted positiveand the output pulses of the second gate circuit are counted minus.

6. A method according to claim 5 wherein aberration of the fringepattern is corrected by modulating the rays entering the said lightdetector with the constant modulating frequency, passing the outputsignal of the first phototransistor (10) through a low frequency bandpass filter (40) to the first wave-forming circuit (12), passing theoutput of the second phototransistor (11) to a low frequency band passfilter (41) to the second wave-forming circuit (14), simultaneouslypassing the modulated fringe signals from the first phototransistor (l0)and the output from the first wave-forming circuit to a third gatecircuit (42), passing the output of the third gate circuit to a phasedetector circuit (43) which compares the modulated frequency to themodulating frequency to determine the nature of the aberration, andbased upon the output of the phase detector circuit (14) mechanicallyrefoc usirgg the fringe pattern.

1. irradiating the said plate with non-parallel coherent rays;
 1. Amethod for measuring the change in thickness of the transparent platecontinuously moving relative to a light detector comprising central andperipheral detectors for detecting the central and one other fringe ofan interferometer diffraction pattern produced by non-parallel coherentrays reflected from the surfaces of the transparent plate comprising thesteps for:
 1. A method for measuring the change in thickness of thetransparent plate continuously moving relative to a light detectorcomprising central and peripheral detectors for detecting the centraland one other fringe of an interferometer diffraction pattern producedby non-parallel coherent rays reflected from the surfaces of thetransparent plate comprising the steps for:
 1. irradiating the saidplate with non-parallel coherent rays;
 2. directing and focusing theresultant two sets of rays which are reflected by the front and rearsurfaces of the said plate to the said light detector, the said two setsof rays producing interference fringe pattern at said detector whichdiverges or converges in accordance with the increase or decrease of theplate thickness;
 3. modulating the rays entering the said light detectorcentral detector by a slight back and forth deflection at a constantmodulation frequency according to a modulation signal;
 4. detecting andcomparing the modulated fringe signal with the modulation signal andproducing a control signal indicative of the phase shift whichcorresponds to the aberration at the center of the fringe pattern fromthe central detector;
 5. in response to said control signal mechanicallydriving means for focusing and correcting the aberration at the centerof the fringe pattern;
 6. differentiating between the divergent andconvergent movement of the fringe pattern by comparing the fringesignals detected by the central and peripheral light detectors; and, 7.reversibly counting the number of fringes moving across the said lightdetector.
 2. directing and focusing the resultant two sets of rays whichare reflected by the front and rear surfaces of the said plate to thesaid light detector, the said two sets of rays producing interferencefringe pattern at said detector which diverges or converges inaccordance with the increase or decrease of the plate thickness;
 2. Amethod for measuring the thickness of the transparent plate according toclaim 1 wherein the rays entering the light detector are modulated byvibrating a half mirror arranged in the path of the said rays.
 3. Amethod for measuring the thickness of the transparent plate according toclaim 1 wherein the rays entering the light detector are modulated by avibrating parallel glass plate arranged in the path of said rays. 3.modulating the rays entering the said light detector central detector bya slight back and forth deflection at a constant modulation frequencyaccording to a modulation signal;
 4. detecting and comparing themodulated fringe signal with the modulation signal and producing acontrol signal indicative of the phase shift which corresponds to theaberration at the center of the fringe pattern from the centraldetector;
 4. A method for measuring the thickness of the transparentplate according to claim 1 wherein the rays entering the said lightdetector are modulated by an electro-optical element arranged in thepath of said rays.
 5. The method according to claim 1 wherein thedifferentiation between the divergence and convergence movement of thefringe pattern is made by detecting with a first phototransistor (10)the intensity of the rays at the center of the fringe pattern and withthe second phototransistor (11) the intensity of a peripheral portion ofthe fringe pattern, the phototransistors being arranged to producesignals of the same frequency space by about pi /2, feeding the signalcreated by the first phototransistor (10) to a first wave-formingcircuit (12) for creating a positive pulse output during a portion ofthe positive part of the input signal, feeding the output of the firstwave-forming circuit (12) to a differentiation circuit (13) for creatinga short positive or negative signal respectively at the start and finishof the pulse output of the first wave-forming circuit, simultaneouslyfeeding the signal created at the second phototransistor (11) to asecond wave-forming circuit (14) to form a pulse output during apositive portion of part of the input signal such that said pulseoverlaps the output pulse of the first wave-forming circuit, feeding theoutput of the first differentiation circuit (13) and the secondwave-forming circuit (14) to a first gate circuit (15) which produces aseries of pulses when the inputs thereto are simultaneously positive,which occurs during divergence of the interference fringe as the outputsignal of first phototransistor leads the output signal the secondphototransistor, feeding the output of the second wave-forming circuitto a phase inverter (16), feeding the output of the firstdifferentiation circuit (13) and the phase inverter (16) simultaneouslyto a second gate circuit (17), which produces a series of signals whenthe inputs thereto are simultaneously positive, which occurs duringconvergence of the interference fringe pattern as the output signal ofthe first phototransistor drags behind the output signal of the secondphototransistor and feeding the outputs of the first and second gatecircuits (15, 17) to a responsive counter (18) wherein the output pulsesof the first gate circuit are counted positive and the output pulses ofthe second gate circuit are counted minus.
 5. in response to saidcontrol signal mechanically driving means for focusing and correctingthe aberration at the center of the fringe pattern;
 7. reversiblycounting the number of fringes moving across the said light detector.